The present invention relates generally to apparatus for measuring soil moisture and, more particularly, to such apparatus wherein the measurements are achieved non-destructively, in situ, and independently of soil color, soil type, and soil texture.
The moisture content of soil, for example, the soil comprising an agricultural field, is a characteristic vital to the efficient use and preservation of the field. Monitoring the moisture content of the soil is important for enabling the estimation of soil water evaporation and run-off rates, for crop yield forecasting, for scheduling field irrigation and for calibrating remote soil moisture sensing instrumentation. While various techniques have been employed in the past to measure soil moisture, for various generally well-known reasons, none has proven altogether satisfactory.
The standard prior art technique for measuring soil moisture consists of sampling a known volume of soil from the field, weighing the sample, and then drying it after which the dried sample is re-weighed. The weight loss occurring during the drying process is thus equal to the mass of water contained in the original soil sample. The gravimetric soil moisture content may then be calculated by dividing the mass of water attributable to the weight loss by the mass of the dried soil, while the volumetric soil moisture content may be derived by dividing water attributable to the weight loss by the volume of the original moist soil sample. Although the foregoing technique has been widely accepted in the industry, it is characterized by a number of severe shortcomings. Initially, the entire measurement process is relatively costly to implement especially on a continuing basis. Moreover, because of the natural inhomogeneity of the soil, the technique is neither very accurate nor very well reproducible. In addition, due to the requirement of a minimum sample volume of a few cubic centimeters for each measurement, a high soil moisture depth resolution as well as an accurate spatial resolution cannot be achieved utilizing this technique.
Other known methods of measuring soil moisture include techniques implementing the phenomena of neutron scattering, gamma ray absorption, microwave emission and microwave scattering. In the known neutron scattering methods, soil moisture is measured over a large volume so that a high soil moisture depth resolution cannot be achieved. Gamma ray methods of measuring soil moisture involve the use of gamma ray emitters and receivers which are inserted into the soil at different locations. A serious limitation characterizing this method is that the equipment must be calibrated differently for different soil types. To implement the microwave emission method of soil moisture measurement, an independent determination of soil temperature must be made. Although such may be accomplished through a measurement of the emitted thermal radiation in the band from 8 to 14 microns, this additional step is undesirable for obvious reasons. In addition, the soil surface roughness affects the microwave signal in a relatively poorly understood manner further detracting from the desireability of utilizing this technique. Another factor detracting from the microwave emission method concerns the resolution along the soil surface (image resolution) which is severly limited by the wavelength of the microwaves and the antenna size, the image resolution being particularly poor when transmission is from aircraft and spacecraft. Yet further, the microwaves emerge from a soil surface layer whose effective thickness depends on the soil moisture itself. Therefore, the effective thickness of the soil layer being measured is largely unknown. And, while the soil moisture profile of a soil sample may be estimatable by performing measurements on the same sample at several different wavelengths, the corresponding soil temperature profile must also be known. To overcome some of these problems, microwave scattering soil moisture measurement techniques have been developed which are only slightly affected by soil temperatures. However, these techniques are in turn very much disturbed by soil roughness and thus of limited utility.
Various indirect methods of measuring soil moisture are also known in the prior art. Numerous examples of these indirect methods may be found in P. J. Geary's publication "Determination of Moisture in Solids", British Scientific Instrument Association, Report M 24 (1956), the Cable Printing and Publishing Company, Ltd., London. The indirect techniques discussed in this publication as well as other known indirect methods suffer from many of the same drawbacks discussed above which characterize known direct methods of measuring soil moisture.